I used @seen {$n,} = (); and not
$seen {$n} = undef; because I used the slice
in the other case as well. I wanted to use as much identical
syntax as possible, and didn't see a reason to special
case adding a single key.

As for using a trailing comma, all I can say is that using
-w is useful. If you use it, you'll see why the
comma was used.